(a) Technical Field of the Invention
The present invention relates to a screw fastening device, and in particular to a screw fastening device that allows efficient mounting and advancing of screws carried on a screw strip along a guide chute to prevent the screws from impacting a target object and damaging the target object and to precisely move the screw strip forward to precisely set the screws at a desired positioning point in a completely fixed condition and also allows for correcting the orientation of the sharp tip of the screw with a guiding and holding device and stably holding a body of the screw to prevent improper orientation of the screw and incorrect screwing thereby effectively and precisely screw the screw to a target object.
(b) Description of the Prior Art
Prior art of the screw fastening device, such as U.S. Pat. No. 7,032,482 B1 to Hoffman, comprises a device body, a stationary base extending from the device body, a screw strip guide rail mounted to a bottom of the stationary base, a screw-driving tool arranged in the device and extending inside the stationary base, a movable base coupled to and extending outside the stationary base and movable with respect to the stationary base, a transmission device arranged in a front end inside the movable base, a connection guide board arranged at a bottom inside the movable base and mountable to the screw strip guide rail, and a positioner arranged at the front end of the movable base. The known device is characterized in that a front end of the connection guide board is coupled to a screw inlet slot at the bottom inside the stationary base by a connection block and a rear end of the connection guide board is directly inserted into the screw strip guide slot. A chute block is mounted to the bottom of the front end of the connection guide board whereby a screw strip is assembleable to and extends beyond a top end of the screw strip guide rail to pass through a chute block on the connection guide board and eventually enters the transmission device of the screw inlet slot to allow the screw strip to be guided by the chute block of the connection guide board, after it disengages from the screw strip guide rail, so as to prevent the screw strip from arbitrary movement. However, when the conventional device is put in a vertical condition and a screw on the screw strip is gradually screwed into a target object and a tail end of the screw strip moves out of and disengages from the chute block, the tail end of the screw strip is out of guidance of the chute block and is in a free suspension condition, under which the tail end of the screw strip, under the action of the gravity thereof, is moved downward to approach the target object. Since the screw fastening device is continuously moved and operated, the tail end of the screw strip in free suspension condition is shaken and arbitrarily moved, causing impact between the screws on the screw strip and a wooden target object and leading to damage of the surface of the target object by sharp tips of the screws and thus damage to aesthetics of the target object. Further, in the conventional device, the screw strip has to pass through the screw strip guide rail and the chute block of the connection guide board in order to enter the transmission device of the screw inlet slot to completely mounting of the screw strip to the device. This is a complicated and troublesome process.
Other known devices, such as U.S. Pat. No. 5,083,483 to Takagi, U.S. Pat. No. 5,339,713 to Hou, U.S. Pat. No. 5,687,624 to Tsuge et al., U.S. Pat. No. 5,889,126 to Fujiyama et al., U.S. Pat. No. 5,988,025 to Sasaki et al., and U.S. Pat. No. 5,988,026 to Reckelhoff et al., all disclose a screw fastening device comprising, as a primary part, a toothed circular body having teeth engageable engaging slots formed on opposite side edges of a screw strip. The toothed circular bodies of the conventional devices are coupled to transmissions of different types to effect advancing of the screw strip. However, this arrangement comprises a variety of parts, which is more likely to cause potential failure of the parts and is more complicated in assembling and maintenance so that reduction of costs is not possible. Further, advancing the screw strip forward with the teeth of the toothed circular body is subject to over-operation or under-operation by an operator with excessive force (screw strip moved too fast) or insufficient force (screw strip moved too slow), whereby the moving speed of the screw strip caused by the teeth of the toothed circular body may not match with the forward pushing operation of the screw-driving tool and thus the screw-driving tool may fail to properly align to the screw for screwing operation. Further, in case the screws carried on the screw strip have an excessive length, the overall weight of the screw strip is unduly increased so that the teeth of the toothed circular body are not able to support the weight, leading to shaking of the screw strip and improper orientation of the screws and imprecise positioning of the screws at desired locations for screwing.
Further, Taiwan Utility Model No. M268145 discloses a screw holding structure that comprises two holding bases that are rotated outward at the time the head of a screw passes therethrough and that are returned to the original position by springs so that the holding bases function to stably hold the screw at the time of screwing operation and also provides opening/closing operation to allow smooth passage of the screw. Although the conventional device provides stable holding of the screw by the holding bases at the time of screwing the screw onto a target object, the two holding bases are pivotally mounted to a casing of a screw pushing device at locations close to an outlet of an operation tube of the screw fastening device so that when the screw is advanced into a screw inlet slot of the casing of the screw pushing device to be driven by the operation tube of the screw fastening device, recesses of the two holding bases can only effectively hold the head of the screw, but the sharp tip of the screw is not held by any members. Apparently, when the screw is driven forward to completely pass the holding bases but the sharp tip does not reach the target object, the screw is in a condition that the screw is not held and supported by the two holding bases and that the screw is not subject to any further support by other members. Any external force applied to the screw makes the sharp tip of the screw that is being forward driven inclined and no longer in proper alignment with the target object. In addition, for screws of excessive length, the weight is increased. Thus, when the screws of excessive length are driven forward to have the head completely pass the holding bases, the sharp tip of the screw is caused by the weight thereof to incline downward, leading to improper alignment to the target object or even jamming of screw. Thus, the known device still suffers the drawback of incapability of supporting and holding the sharp tip of the screw and thus being easy to incline when the screw is driven forward, leading to improper alignment of the screw to the target object.
Thus, it is desired to provide a screw fastening device to overcome the above discussed problems associated with the conventional devices.